This invention relates generally to an internal combustion engine inlet air system and more particularly to an inlet air system driven by gas turbines.
It is conventional for supercharged or turbocharged engines to have a system for cooling the inlet air to approximately the temperature of the ambient air adjacent the engine. Otherwise, the inlet air would be at a higher temperature due to the heat of compression, which could result in poor engine operation. It is also recognized that engines benefit from the inlet air being cooled below the ambient air temperature. In particular, refrigeration below ambient can provide a combination of reduced specific NO.sub.x emissions, increased specific power and increased efficiency. The method most generally used for reducing specific NO.sub.x emissions, leaning the engine mixture, causes a reduction in specific power and may cause a loss of efficiency.
Natural gas is moved in large quantities through pipelines by means of compressor stations along the pipelines. These compressor stations use reciprocating or gas turbine engines which are fueled by the pipeline gas itself. As part of the national and international effort to reduce air pollution, the exhaust emissions from these engines are coming under increasingly strict limits. Reducing NO.sub.x emissions from reciprocating engines has proved to be fairly difficult, and in some cases, existing installed engines must be shut down, resulting in an economic loss to the operator.
A primary strategy for NO.sub.x reduction in reciprocating engines that has been identified by recent research is to operate the engine with a substantially leaner fuel/air mixture. If the engine air flow is maintained constant and the fuel flow is reduced to lean the mixture, there will be a loss of engine power output roughly proportional to the reduction in fuel flow. Therefore, to make up this loss in engine power, it is desirable to increase the air flow through the turbocharger compressor. However, when the mixture is leaned with constant air flow, the engine exhaust temperature decreases, and hence the power available in the turbocharger turbine will decrease. Therefore, often the turbocharger turbine cannot produce the additional power. In this case, the desired NO.sub.x reduction is achieved, but at the cost of reduced power available from the engine.
In some cases, the reduction in engine exhaust temperature is such that the turbocharger turbine cannot maintain the original air flow through the compressor. Thus, the compressor air flow falls as the fuel flow is reduced, and the desired lean fuel/air ratio can never be reached. In this case, the desired NO.sub.x reduction cannot be achieved and the engine may have to be shut down.
The foregoing illustrates limitations known to exist in present inlet air systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.